JP2005030246A

JP2005030246A - Exhaust emission control device

Info

Publication number: JP2005030246A
Application number: JP2003194164A
Authority: JP
Inventors: Takeshi Tanabe; 健田辺; Kinichi Iwachido; 均一岩知道
Original assignee: Mitsubishi Motors Corp
Current assignee: Mitsubishi Motors Corp
Priority date: 2003-07-09
Filing date: 2003-07-09
Publication date: 2005-02-03

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device improving NOx and HC purifying efficiency regardless of a change in temperature of a catalyst. <P>SOLUTION: This device is provided with a NOx catalyst C1 arranged in the exhaust passage Re of an internal combustion engine 1, absorbing NOx in exhaust when an exhaust air-fuel ratio is lean, and releasing absorbed NOx when the exhaust air-fuel ratio is a stoichiometric air-fuel ratio or rich to reduce; a NOx releasing means A1 releasing NOx in the NOx catalyst C1 by making the exhaust air-fuel ratio; into a rich air-fuel ratio, a reducing agent supplying means A2 adding and supplying a reducing agent to reduce released NOx when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich, a temperature sensor 28 outputting the catalyst temperature Tc of the NOx catalyst C1; and a purge control means A3 changing over and controlling addition and supplying of the reducing agent by the reducing agent supplying means A2 according to the catalyst temperature Tc according to the catalyst temperature Tc. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、排気ガス浄化装置、特に、ＮＯｘ、ＨＣを効率よく浄化できる排気ガス浄化装置に関する。
【０００２】
【従来の技術】
環境保全のため車両の排気ガス浄化規制がより強化されてきており、これに伴って内燃機関の運転域の変化にかかわらず、触媒の浄化性能をより高くすることが要求されてきている。
【０００３】
ところで、燃費の向上を図るため、酸素過剰雰囲気中（リーン空燃比）での燃焼を可能とした希薄燃焼内燃機関が実用化されている。この希薄燃焼内燃機関（リーンバーンエンジン）では、リーン燃焼時に排ガス中のＮＯｘを吸蔵して排気中のＮＯｘを浄化するＮＯｘ触媒が採用されている。このＮＯｘ触媒は、酸素過剰雰囲気中（リーン空燃比）においてはＮＯｘを触媒上に吸蔵させることにより排気中のＮＯｘを浄化し、酸素濃度が低下すると（理論空燃比またはリッチ空燃比）付着したＮＯｘを放出する機能を有している。つまり、ＮＯｘ触媒は酸素濃度過剰雰囲気では、排気中のＮＯｘから硝酸塩を生成し、これによりＮＯｘを吸蔵する一方、酸素濃度が低下した雰囲気では、ＮＯｘ触媒に吸蔵した硝酸塩と排気中のＣＯとを反応させて炭酸塩を生成し、これによりＮＯｘを放出させるようになっている。
【０００４】
ＮＯｘ触媒は、リーン運転中の酸素過剰雰囲気中ではＮＯｘを触媒上に吸蔵させることになるが、リーン運転を連続して行いＮＯｘの吸蔵量が飽和量に達した時には排気中のＮＯｘの大部分が大気に排出されることになる。そこで、ＮＯｘの吸蔵量が飽和量に達する前に、空燃比を理論空燃比またはリッチ空燃比に切り換え、排気を酸素濃度が低下した雰囲気にしてＮＯｘを放出還元させ、ＮＯｘ触媒のＮＯｘ吸蔵能力を回復させるようにしている。
【０００５】
この運転域では、ＮＯｘ吸蔵能力を回復させるために機関の空燃比を理論空燃比またはリッチ空燃比に切り換えて（ＣＯを発生させ排気中、即ち、ＮＯｘ触媒に供給して）ＮＯｘを放出還元する場合、供給されたＣＯは、吸蔵されたＮＯｘを放出するために一部のＣＯが消費され、放出されたＮＯｘを還元するために残りのＣＯが消費される。
【０００６】
ところが、ＮＯｘ放出速度は、機関の空燃比がリッチ側に移行するにつれて（ＣＯ量が増加するにつれて）増大し、特に、理論空燃比近傍からＣＯ量が増大するのに応じて触媒上から放出されるＮＯｘ放出量が増加する。このため、増加したＮＯｘ放出量に対して放出したＮＯｘを還元するだけの還元剤（ＮＯｘ放出に寄与しなかった残りのＣＯ及びＨＣ等）が少ないと、排ガス中に残留した放出ＮＯｘが還元されずそのまま大気中へ放出されてしまう。
【０００７】
このように空燃比がリーンよりリッチ側に移行し、理論空燃比やリッチ側への移行が僅かな運転が継続するのみの制御では、ＮＯｘ放出量が急増し、還元されるＮＯｘ＜放出されるＮＯｘ、という関係は変わらず、ＮＯｘ触媒で還元されずに下流に放出されてしまう。
【０００８】
この解決法として、ＮＯｘ触媒のＮＯｘ吸蔵量が飽和量に達する前に触媒内の排気空燃比を早期にリッチ化すべく通常の吸気行程噴射に加えて、膨張噴射を短時間追加するパージ運転を行うことが知られている。このパージ運転を行なうことで、排気ガス中に還元剤を増量供給するよう制御でき、ＮＯｘ触媒からのＮＯｘを放出させ、還元し、ＮＯｘ浄化性能を向上させることができ、その一例が特開平１１―２１１４号公報（特許文献１）にも記載されている。
【０００９】
例えば、直噴リーンバーンエンジンでは、リーン運転中のＮＯｘトラップ性能と、リーン以外の運転中のＮＯｘ、ＨＣ、ＣＯ浄化性能を高く保持する必要がある。そこで、図８に示すように、排気路に沿って縦配列でＮＯｘ触媒１００と、三元触媒１１０を配備した排気ガス浄化装置１２０が用いられる。なお、特許文献１に開示の排気ガス浄化装置では排気路に沿って前段三元触媒、ＮＯｘ触媒、後段三元触媒をこの順に縦に順次配備する構成を開示している。
【００１０】
【特許文献１】
特開平１１―２１１４号公報
【００１１】
【発明が解決しようとする課題】
図８や特許文献１に開示の排気ガス浄化装置では、ＮＯｘ浄化性能を向上させることができる。しかし、この場合、少なくともＮＯｘ触媒が活性化していることが必要となり、活性化していない低温運転域でＮＯｘパージ処理のため、不図示の還元剤供給手段よりＨＣの還元剤が供給されると、このＨＣは還元にほとんど寄与せず、ＮＯｘ排出量を的確に低減することができない。しかも、そのまま排出されるＨＣがＨＣ排出濃度を増加し、無駄な燃料消費を招くこととも成る。
【００１２】
ところで、本願発明者らはＮＯｘ触媒が雰囲気温度の変化に応じてそのＮＯｘ浄化効率やＨＣ浄化効率の変化特性を探究しておく必要があると考え、実験を行なった。ここでは図８のタンデム式の排気ガス浄化装置を用い、不図示のエンジンの燃料噴射弁による燃料噴射駆動をリーン時噴射パターンと、リッチ時噴射パターンと、パージ時噴射パターンとに切換え調整し、これにより空燃比をリーン、リッチ、ＮＯｘパージに切換え制御を行なった。
【００１３】
ここで、触媒温度を高低変化させ、各温度でのＮＯｘ浄化効率とＨＣ浄化効率とを測定し、図９（ａ）、（ｂ）に示す特性が明らかと成った。
図９（ａ）に示すように、排気ガス浄化装置のＮＯｘ浄化効率は、還元剤添加によるＮＯｘパージ処理の有無（符号ａがパージ、符号ｂがパージなし）にかかわらず、触媒温度が３５０℃までは温度の増加により活性化がほぼ同一の浄化効率で進み、３５０℃を越えると、触媒自体の特性でＮＯｘトラップ性能が低下している。この場合、３５０℃を越えた運転域ではＮＯｘパージ処理を行なう方が行なわない場合よりＮＯｘ浄化効率が向上することが明らかとなった。
【００１４】
図９（ｂ）に示すように、排気ガス浄化装置のＨＣ浄化効率は、ＮＯｘパージ処理の有無（符号ａがパージ、符号ｂがパージなし）にかかわらず、触媒温度が低温側より高まるのに応じて上昇している。特に３３０℃を超えるまでの低温域でのＨＣ浄化効率は低下しており、ここで還元剤（ＨＣ）を追加するとＨＣ浄化効率がより悪化している。３３０℃を上回ると、還元剤（ＨＣ）が十分に消費されＨＣ排出量が抑えられることが明らかと成った。
【００１５】
上述のように排気ガス浄化装置はＮＯｘ浄化効率を高めるべく還元剤を追加しても、３５０℃近傍までは還元機能が的確に働かず、しかも、ＨＣ浄化効率は３３０℃を超えるまでは還元剤（ＨＣ）を追加することでＨＣ浄化効率がより悪化していることより、還元剤（ＨＣ）の追加は低温側域では抑制することが好ましい。一方、３５０℃以上では還元剤の追加によりＮＯｘ浄化効率が高められ、３３０℃を超えると還元剤（ＨＣ）が十分に消費されてＨＣ浄化効率が還元剤（ＨＣ）を添加した場合の方が向上することより、高温側域では還元剤（ＨＣ）の追加が好ましいことが判明した。
【００１６】
本発明は、以上のような経緯に基づきなされたものであり、排気ガス浄化装置における還元剤添加を触媒温度に応じて調整することで、ＮＯｘ浄化効率及びＨＣ浄化効率を触媒温度の変化にかかわらず向上させることができる排気ガス浄化装置を提供することを目的とする。
【００１７】
【課題を解決するための手段】
請求項１の発明は、内燃機関の排気路に設けられ排気空燃比がリーン空燃比である時に排気中のＮＯｘを吸収し、排気空燃比が理論空燃比又はリッチ空燃比であるときに、上記吸収したＮＯｘを放出して還元処理するＮＯｘ触媒装置と、上記排気空燃比をリッチ空燃比とすることで上記ＮＯｘ吸収触媒中のＮＯｘを放出させるＮＯｘ放出手段と、上記排気空燃比が理論空燃比又はリッチ空燃比であるときに、上記放出されたＮＯｘを還元する還元剤を追加供給する還元剤供給手段と、上記ＮＯｘ触媒装置の触媒温度を出力する温度検出手段と、上記還元剤供給手段による還元剤の追加供給を上記触媒温度に応じて切換え制御するパージ制御手段と、を具備することを特徴とする。
このように、ＮＯｘ放出手段により排気空燃比をリッチ空燃比とした際に、パージ制御手段が還元剤の追加供給を触媒温度に応じて切換え制御することで、リッチ空燃比であっても、還元剤の追加供給を触媒温度に応じて停止することが可能となり、ＮＯｘ触媒装置が活性化される前の排気ガス中のＨＣ濃度の増加を防止できる。
好ましくは、パージ制御手段が還元剤の追加供給を膨張行程で行なうことがよい。この場合より的確に還元剤の追加供給を行なえ、ＮＯｘ吸収触媒中のＮＯｘ放出、放出ＮＯｘの還元を促進できる。
【００１８】
請求項２の発明は、請求項１記載の排気ガス浄化装置において、上記パージ制御手段は上記触媒温度が設定温度未満では上記還元剤供給手段による還元剤の追加供給を停止し、設定温度以上で還元剤の追加供給を行なうことを特徴とする。
このように、ＮＯｘ放出手段により排気空燃比をリッチ空燃比とした際に、触媒温度が設定温度未満では還元剤の追加供給を停止し、ＮＯｘ触媒装置が活性化される前の排気ガス中のＨＣ濃度の増加を防止し、設定温度以上で還元剤の追加供給を行って、ＮＯｘ触媒装置から放出されるＮＯｘの還元浄化を的確に行なうことができる。
【００１９】
請求項３の発明は、請求項１又は２記載の排気ガス浄化装置において、上記パージ制御手段は上記還元剤の追加供給量を上記触媒温度の増加に応じて増加させることを特徴とする。
このように、ＮＯｘ放出手段により排気空燃比をリッチ空燃比とした際に、還元剤の追加供給量を触媒温度の増加に応じて増加させるので、低温時に無駄な還元剤供給を抑えて排気ガス中のＨＣ濃度の増加を防止でき、高温に移動するに従い還元剤の追加供給量を増加させるので、燃料噴射の切換えがスムーズになされ、しかも、低温側で比較的早く追加燃料供給でき、早期の活性化にも寄与できる。
【００２０】
請求項４の発明は、請求項１、２又は３に記載の排気ガス浄化装置において、上記ＮＯｘ触媒装置の排気路下流側に三元触媒が配置されたことを特徴とする。
このように、ＮＯｘ触媒装置の排気路下流側の三元触媒はＮＯｘ触媒装置側で処理されていないＮＯｘ、ＣＯ、ＨＣを処理して、排気浄化触媒装置全体の浄化効率を向上させることができる。
【００２１】
【発明の実施の形態】
図１は本発明の一実施形態としての排気ガス浄化装置Ｍと、同装置を装備する内燃機関を示した。内燃機関は筒内噴射型４サイクル多気筒ガソリンエンジン（以後、単にエンジン１と記す）である。エンジン１はシリンダブロック２内のシリンダ３と同シリンダ内で上下摺動するピストン４と、シリンダブロック２に重なり一体的に結合されるシリンダヘッド５とで囲むように燃焼室６を形成しており、同燃焼室６は気筒数だけ（図１には１つのみ示す）配備される。燃焼室６には電磁式の燃料噴射弁７が取り付けられる。燃料噴射弁７は吸気行程で燃焼室６に燃料噴射する吸気行程噴射モード、圧縮行程で燃料噴射する圧縮行程噴射モード、膨張行程で燃料噴射する膨張行程噴射モードでの各燃料噴射が実施可能に形成されている。
【００２２】
エンジン１は、理論空燃比（ストイキ）での運転やリッチ空燃比での運転（リッチ空燃比運転）の他、リーン空燃比での運転（リーン空燃比運転）が実現可能となっており、特に、圧縮行程噴射モードでは、吸気行程でのリーン空燃比運転よりも大きな空燃比となる超リーン空燃比での運転が可能となっている。
各燃焼室６には相互に干渉しない位置に燃料噴射弁７、吸気弁８、排気弁９及び点火プラグ１１が取り付けられる。
【００２３】
シリンダ３内のピストン７はそのピストンの頂面に半球状に窪んだキャビティ１２が形成されている。キャビティ１２は吸入ポート１３と協働し、図１において時計回りの逆タンブル流Ｓを発生させるようになっている。シリンダヘッド５には、各気筒毎に略直立方向に吸気ポートが形成され、各吸気ポート（図１には１つのみ示す）１３には吸気路Ｒｉを形成する吸気マニホールド１４、サージタンク１５、スロットル弁１６を有した吸気管１７、エアクリーナ１８、大気側開口１９がこの順に連結され、大気を吸気路Ｒｉに沿って吸入可能に形成される。
【００２４】
スロットル弁１６はドライブバイワイヤ（ＤＢＷ）方式の電動弁であり、スロットル開度θｔｈを検出するスロットルポジションセンサ２１が設けられている。エンジン１には、クランク角を検出するクランク角センサ２２が設けられ、クランク角センサ２２はエンジン回転速度Ｎｅを検出可能となっている。
シリンダヘッド５には、各気筒毎に略水平方向に排気ポート２３が形成され、各排気ポート（図１には１つのみ示す）２３には排気路Ｒｅを形成する排気マニホールド２４、排気管（排気通路）２５、排気ガス浄化装置Ｍの要部をなす排気浄化触媒装置２６、下流側排気管２７、図示しないマフラーがこの順に連結され、排気を排気路Ｒｅに沿って外部に排出可能に形成される。
【００２５】
排気浄化触媒装置２６の直上流の排気管２５にはＮＯｘ触媒装置の触媒温度を推測するために排気温度を検出する温度検出手段たる温度センサ２８が設けられ、更にその上流側には空燃比Ａ／Ｆを検出する空燃比センサ３０が設けられている。
図１に示すように、排気浄化触媒装置２６は、排気管２５及び下流側排気管２７に連続するよう内径を拡大させた形状の筒状のケーシング２９と、ケーシング２９内にずれなく縦向きに前後配備されるＮＯｘ触媒Ｃ１と三元触媒Ｃ２とで形成される。
【００２６】
図１、図２に示すように、前後の、軸方向に貫通する多数の貫通孔３１を持つ前後の担体３２、３３と、各貫通孔３１を区画する内面に形成された耐火性無機酸化物の担持層３４と、担持層３３に保持された貴金属の触媒成分３５とを有する。
各担体３２、３３は主成分がアルミナで、これにより各貫通孔３１の内面近傍の担持層３４を含むハニカム構造体全体が形成される。前の担体３２の担持層３４にはＮＯｘトラップ剤が混入され、内面には還元促進用の貴金属の触媒成分３５が担持される。後の担体３３の担持層３４には酸化促進剤が混入され、内面には貴金属の触媒成分３５が担持される。
【００２７】
図１、図２の担体３２はその担持層３４にＮＯｘトラップ剤として耐熱性が高く、ＮＯｘトラップ性能の高いＫ（カリウム）を主体とするアルミナを採用して形成され、担体３３はその担持層３４にＨＣ、ＣＯ酸化処理性能の高いＢａ（バリウム）を主体とするアルミナを採用して形成される。
更に、担体３２、３３の各担持層３４の内壁面には、ＮＯｘ還元処理、ＨＣ酸化処理性能の高い貴金属の触媒成分３５としてＰｔ（プラチナ）が担持される。なお、ここでの貴金属の触媒成分３５はＮＯｘ還元処理、ＨＣ酸化処理性能を高める上で多い方が好ましいが、ＮＯｘ触媒Ｃ１側ではＮＯｘトラップ剤（カリウム）のＮＯｘの吸着、放出、還元の機能を抑制しない範囲で適量が担持されることとなる。
【００２８】
このような排気浄化触媒装置２６の上流側のＮＯｘ触媒Ｃ１は、酸素過剰雰囲気となる排気空燃比がリーン空燃比のときにおいてＮＯｘを一旦吸蔵させ、酸素濃度が低下して排気空燃比が理論空燃比またはリッチ空燃比のとき、即ち、ＣＯの存在する還元雰囲気中においてＮＯｘを放出してＮ２（窒素）等に還元させるＮＯｘ放出還元機能を持つものである。
下流側の三元触媒Ｃ２は理論空燃比およびリッチの雰囲気で排気ガス中のＣＯ、ＨＣを酸化し、ＮＯｘを還元して浄化する三元機能を有する。なお、三元触媒Ｃ２はＮＯｘ触媒Ｃ１から放出され、同部で還元しきれなかったＮＯｘの還元も行なう。
【００２９】
車両にはエンジン制御手段であるコントローラ（以後単にＥＣＵ３６と記す）が設けられ、ＥＣＵ３６には、入出力装置、制御プログラムや制御マップ等の記憶を行う記憶装置、中央処理装置及びタイマやカウンタ類が備えられている。ＥＣＵ３６によってエンジン１を含めた排気浄化触媒装置２６の制御が実施される。各種センサ類の検出情報はＥＣＵ３６に入力され、ＥＣＵ３６は各種センサ類の検出情報に基づいて、燃料供給制御や点火制御を実行し、即ち、燃料噴射モードや燃料噴射量を始めとして点火時期等を決定し、燃料噴射弁７や点火プラグ１１等を駆動制御する。
【００３０】
ＥＣＵ３６の燃料供給制御手段Ａ０は、定常運転域にあると圧縮行程中期以降に燃料を噴射して点火プラグ１１の近傍のみに少量の燃料を集め、点火プラグ１１の近傍のみを理論空燃比又はリッチな空燃比Ａ／Ｆとすることで、安定した層状燃焼（層状超リーン燃焼）を実現しながら燃料消費を抑制する。また、高出力を得る場合には、燃料噴射弁７からの燃料を吸気行程に噴射することにより燃焼室６全体に均質分散させ、燃焼室６内を理論空燃比（略Ａ／Ｆ：１５）やリーン空燃此の混合気状態にさせて予混合燃焼を行ない、高出力を得るようにしている。
【００３１】
この燃料供給制御手段Ａ０では、スロットルポジションセンサ２１からのスロットル開度θｔｈとクランク角センサ２２からのエンジン回転速度Ｎｅとに基づいてエンジン負荷に対応する目標筒内圧、即ち、目標平均有効圧Ｐｅが求められ、更に、この目標平均有効圧Ｐｅとエンジン回転速度Ｎｅとに応じてマップ（図示せず）より燃料噴射モードが設定される。
【００３２】
例えば、目標平均有効圧Ｐｅとエンジン回転速度Ｎｅとが共に小さいときは、燃料噴射モードは圧縮行程噴射モードとされて燃料が圧縮行程で噴射される。一方、目標平均有効圧Ｐｅが大きくなり、あるいはエンジン回転速度Ｎｅが大きくなると燃料噴射モードは吸気行程噴射モードとされる。そして、目標平均有効圧Ｐｅとエンジン回転速度Ｎｅとから各燃料噴射モードでの制御目標となる目標空燃比Ｏ_Ａ／Ｆが設定され、例えば、リーン空燃比ｓｅ１では図４中のパルス噴射ＰＬでの運転がなされ、リッチ空燃比ｓｅ２では図４中に破線で示すパルス噴射ＰＨでの運転がなされ、適正量の燃料噴射量がこの目標空燃比Ｏ_Ａ／Ｆに基づいて決定される。
【００３３】
ところで、排気浄化触媒装置２６のＮＯｘ触媒Ｃ１では、排ガス中の空燃比がリーン空燃比のときに、排気中のＮＯｘが硝酸塩として吸蔵されて排気の浄化が行われる。一方、排ガス中の空燃比が理論空燃比またはリッチ空燃比のときに、ＮＯｘ触媒Ｃ１に吸蔵した硝酸塩と排気中のＣＯとが反応して炭酸塩が生成されると共にＮＯｘが放出される。従って、ＮＯｘ触媒Ｃ１へのＮＯｘの吸蔵が進むと、空燃比のリッチ化を行うリッチパージを行なって、酸素濃度を低下させ、ＮＯｘ触媒Ｃ１へＣＯを供給し、ＮＯｘ触媒Ｃ１からＮＯｘを放出還元させてＮＯｘ吸蔵機能を維持することが可能である。
【００３４】
そこで、ＥＣＵ３６には上述の燃料供給制御手段Ａ０に加えＮＯｘ放出手段Ａ１と還元剤供給手段Ａ２とパージ制御手段Ａ３しての機能が備えられている。ＮＯｘ放出手段Ａ１は排気中の酸素濃度を低下させて（リッチ空燃比）、ＮＯｘ触媒Ｃ１中からＮＯｘを放出させる。還元剤供給手段Ａ２は目標空燃比Ｏ_Ａ／Ｆが理論空燃比又はリッチであるときに、放出されたＮＯｘを還元する還元剤（ＨＣ）を追加供給するパージ用燃料噴射ＰＰを指定する。このため、パージ指令時にはＥＣＵ３６はＮＯｘ放出手段Ａ１がリッチ空燃比ｓｅ２を指定すると共に還元剤供給手段Ａ２が追加燃料であるパージ用燃料噴射ＰＰを指定するので超リッチ空燃比ｓｅ３が指定されることとなり、図４中のパルス噴射ＰＨ＋ＰＰでの運転がなされる。パージ制御手段Ａ３は高温センサ２８より得られた触媒温度Ｔｃに応じて還元剤供給手段Ａ２による還元剤（ＨＣ）の追加供給するパージ用燃料噴射ＰＰの実施を切換え制御する。
【００３５】
ここでＮＯｘ放出手段Ａ１は、ＮＯｘ触媒Ｃ１からＮＯｘを放出させるべくリッチパージ時間Ｔｒ（図４参照：例えば２ｓｅｃ）の間目標空燃比をリッチ空燃比として出力し、その後、リーン継続時間Ｌｔ（図５参照：例えば、３０ｓｅｃ）の間排気空燃比をリーン空燃比に戻すというリッチパージ機能を備えている。
パージ制御手段Ａ３は還元剤の追加供給（パージ用燃料噴射ＰＰ）を触媒温度Ｔｃに応じて切換え制御する。即ち、パージ制御手段Ａ３は触媒温度Ｔｃが設定温度Ｔｃ１未満では還元剤供給手段Ａ２による還元剤の追加供給を停止し、設定温度Ｔｃ１以上で還元剤の追加供給を行なう。
【００３６】
ここで、設定温度Ｔｃ１は３００℃に設定した。この理由は、図９（ａ）、（ｂ）で説明したように、排気浄化触媒装置２６のＮＯｘ触媒Ｃ１は、雰囲気温度が３５０℃までは還元機能が的確に働かず、しかも、還元剤（ＨＣ）添加は３３０℃を超えるまではＨＣ浄化効率をより悪化していること、更に、３５０℃以上で還元剤（ＨＣ）の追加によりＮＯｘ浄化効率が高められ、３３０℃を超えるとＨＣ浄化効率が還元剤（ＨＣ）を添加した場合の方が向上するとの各点、及び早期活性を図る点を考慮したものである。なお、この設定温度Ｔｃ１＝３００℃は、ＮＯｘ触媒Ｃ１の容量、ＮＯｘ触媒Ｃ１を構成する担体３２、担持層３４のトラップ剤、貴金属の触媒成分３５によって増減変動幅があり、３００〜３５０℃の範囲で適宜修正されることとなる。
なお、ＮＯｘ触媒Ｃ１下流の三元触媒Ｃ２はその触媒温度がＮＯｘ触媒Ｃ１にほぼ連動し、ほぼ同時に活性化する。
【００３７】
上述した排気浄化触媒装置２６の作用を説明する。
ここで、リーンパルス噴射ＰＬが実行されると、ＮＯｘ触媒Ｃ１は担持層３４でＮＯｘを吸増し、ＮＯｘ浄化を実行する。更に、リッチパルス噴射ＰＨが実行された場合、三元触媒Ｃ２の前側のＮＯｘ触媒Ｃ１は排気ガスが理論空燃比近傍に達すると、そこからＣＯを放出し始めるが、放出したＮＯｘを還元するだけの還元剤（残りのＣＯ及びＨＣ等）が少ないため、放出されたＮＯｘ＞還元されるＮＯｘ、となり、ＮＯｘは三元触媒Ｃ２に流動し、ここでＣＯ、ＨＣと共に浄化される。
【００３８】
これに対し、リーン運転の積算時間がリーン継続時間Ｌｔを越えると、ＥＣＵ３６のパージ制御手段Ａ３は触媒温度Ｔｃが設定温度Ｔｃ１＝３００℃を上回る場合のみ、ＮＯｘ触媒が活性化していると判断し、ＮＯｘ放出手段Ａ１、還元剤供給手段Ａ２の駆動を許容する。ここでＮＯｘ放出手段Ａ１は目標空燃比をリッチ空燃比にしてリッチパルス噴射ＰＨ指示を行なう。しかもリッチパージ信号を受けた還元剤供給手段Ａ２は還元剤供給が膨張行程まで続くようパージ用燃料噴射ＰＰの指示をする。これにより、燃料噴射駆動回路３７は燃料噴射弁７をリッチパルス噴射ＰＨに追加してパージ用燃料噴射ＰＰの駆動を連続させたパターン（図４のパルス噴射参照）での燃料噴射駆動を行なうよう、カウンタをセットし、駆動させ燃料増された噴射駆動を行なう。
【００３９】
これにより膨張行程で噴射され、燃焼に寄与しない未燃のＨＣ（還元剤）が燃焼室より排気路Ｒｅに排出されてＨＣ、ＣＯの還元剤となり、これによってＮＯｘ触媒Ｃ１から放出されるＮＯｘを急増させ（図５のＮＯｘ濃度参照）、これを貴金属の触媒成分３５で十分に還元し、三元触媒Ｃ２でも比較的多量の貴金属の触媒成分３５でＮＯｘ及びＣＯ、ＨＣを浄化する。従って、ＮＯｘ及びＣＯ、ＨＣの大気放出を抑えることができ、ＮＯｘ触媒Ｃ１から放出されたＮＯｘがそのまま大気に放出されてしまうといった問題もなくなる。
【００４０】
上述したＮＯｘの放出還元制御であるＮＯｘパージ制御について図６のフローチャートに基づいて説明する。
ＥＣＵ３６の不図示のメインルーチン内での燃料供給制御の途中でＮＯｘパージ制御に達するとする。
【００４１】
なお、不図示のメインルーチンの燃料供給制御において、ＥＣＵ３６が指示する噴射パターンに沿って燃料噴射駆動回路３７が噴射時期、噴射駆動時間を不図示のカウンタでカウントし、燃料噴射弁７を燃料噴射作動させる。例えば、目標空燃比Ｏ_Ａ／Ｆがリーン空燃比（図４に示す符号ｓｅ１）の場合、リーン運転指令が出され、燃料噴射駆動回路３７はリーンパルス噴射ＰＬを行なうよう、不図示のカウンタをセットし、図４に示すリーンパルス噴射ＰＬでの駆動を行なう。更に、目標空燃比Ｏ_Ａ／Ｆがリーン空燃比よりストイキオ及びリッチ空燃比に変更すると、ストイキオ及びリッチパルス噴射ＰＨを行なうよう、カウンタをセットし、図４に示すリッチパルス噴射ＰＨを行なっている。この際、図４に示す破線で示す位置近傍での空燃比Ａ／Ｆが継続する。
【００４２】
メインルーチン内での燃料噴射制御手段Ａ０での途中で、パージ噴射制御指示が有ると、図６のパージ噴射処理に達する。
ここでは、制御の最初の時点であると、ステップｓ１においてパージ中フラグＰＦＬＧ（後述する）が「０」でステップＳ２に進む。なお、ＰＦＬＧ＝１ではステップｓ６に進む。ステップＳ２では、三元触媒Ｃ２の温度ＴがＴｓ（活性温度）以上になったか否かが判断され（高温センサ２８で検出される排気温度値で推定）、ＮＯｘ触媒Ｃ１及び三元触媒Ｃ２の温度ＴがＴｓ以上であると判断された場合（即ち、三元触媒Ｃ２が活性時にある場合）、ステップＳ３でリーンモードの継続時間ｔ１がリーン継続時間Ｌｔ以上か否かが判断される。
【００４３】
なお、リーン継続時間Ｌｔは、次の（１）〜（３）の要件を満たすように設定される。
（１）リーン運転を継続する内にＮＯｘがＮＯｘ触媒Ｃ１に吸蔵しきれなくなり、大気中に放出される状態までの時間以下であることが望ましい。
（２）リーン時間が長く続くと（三元）触媒温度が低下するので、リーン継続時間Ｌｔは所定値以下であることが好ましい。
（３）ＮＯｘパージ立ち上げ時においてリーンからストイキに達するに要するＮＯｘパージ無駄時間ｔｕ（図４参照）が増加し燃費が悪化することを防止するため、リーン継続時間Ｌｔは所定値以下であることが好ましい。
【００４４】
以上の３つの条件により最適なリーン継続時間Ｌｔとしてここでは３０秒を設定している。
ステップＳ４に達すると、ここでは触媒温度Ｔｃが設定温度Ｔｃ１＝３００℃を上回るか否か判断し、上回る場合のみ、即ち、ＮＯｘ触媒が活性化している場合はステップｓ５に進む。
【００４５】
ステップＳ５ではＮＯｘ触媒が活性化し、ＮＯｘ触媒Ｃ１に吸蔵されたＮＯｘを放出し還元する制御（ＮＯｘパージ）の開始条件が成立したとして、ここでパージ用燃料噴射ＰＰを行なうリッチパージ時間Ｔｒ及びパージ用燃料噴射ＰＰの噴射時間であるパルス噴射時間Ｔｉを設定する。
【００４６】
リッチパージ時間Ｔｒは、排気流量、例えば吸入空気量Ｑａによるマップ値と、ＮＯｘ触媒Ｃ１の劣化度合い、例えば走行距離Ｋｍ（積算距離検出器３６より）のマップ値との積に基づいて設定される。リッチパージ時間Ｔｒは、例えば１秒から５秒程度に設定され、走行距離が長くなる程、図４に示したようにＮＯｘ放出特性線図におけるＮＯｘ放出速度が速い領域ｎが狭まることに応じて短くし、これにより、燃費の悪化及び未燃ＨＣ、ＣＯの放出を抑制する。
【００４７】
パルス噴射時間Ｔｉは、例えば０．１秒から１秒程度に設定され、走行距離が長くなる程、即ち、劣化度合いが高くなる程１／２程度までの短い時間になるように設定される。パルス噴射時間Ｔｉを短くするのは、ＮＯｘ触媒１５の劣化度合いが高くなると、ＮＯｘ放出特性が変化し、ＮＯｘ放出速度が速い領域ｎ（図４参照）が狭まるため、還元剤も少量でよいためであり、燃費の悪化及び未燃ＨＣの放出を抑制している。
【００４８】
ステップＳ５でリッチパージ時間Ｔｒ及びパルス噴射時間Ｔｉを設定し、カウントが開始された後、ステップＳ６でリッチパージ時間Ｔｒに達したか否か判断し、達する前はステップＳ７でパルス噴射時間Ｔｉによるパージ用燃料噴射ＰＰを指示する。
この結果、燃料噴射駆動回路３７は、リッチパージ時間Ｔｒに達するとリッチパルス噴射ＰＨの直後にパージ用燃料噴射ＰＰの駆動を加えて連続させたパターン（図４のパルス噴射量参照）で燃料噴射弁７を駆動させ、パルス噴射ＰＨ＋ＰＰを連続させた燃料噴射駆動を行なう。
【００４９】
これにより、燃焼室６には還元剤が追加される。即ち、リッチパルス噴射ＰＨの後のパージ用燃料噴射ＰＰは膨張行程の中期から排気行程の初期、特に膨張行程の後期が望ましく、膨張行程の後期に燃料を追加することで、燃焼することなく未燃の燃料（還元剤）が排気通路に供給され、これがＮＯｘ触媒Ｃ１に吸着していたＮＯｘを急激に放出させ（図４、図５のＮＯｘ濃度参照）、放出されたＮＯｘを貴金属の触媒成分３５の働きで還元処理する。
【００５０】
この場合、図５のＮＯｘ濃度線図に示すように、入口の触媒温度Ｔｃが３００℃の場合の出口ＮＯｘ濃度（１点鎖線で示した）は比較的過度な濃度上昇を示さないことよりＮＯｘ浄化効率が高レベルに保持されていることが明らかである。これに対し、入口の触媒温度Ｔｃが４５０℃の場合の出口ＮＯｘ濃度（実線で示した）は３００℃の場合より濃度が上昇しており、ＮＯｘ浄化効率が低下しているが、、図５のＣＯ濃度（％）線図に示すように、入口の触媒温度Ｔｃが３００℃の場合の出口ＣＯ濃度（％）は極低レベルで、４５０℃の場合（実線で示した）はパージ運転時に所定濃度を示している。
【００５１】
更に、図５のＨＣ濃度（％）線図に示すように、入口の触媒温度Ｔｃが３００℃の場合の出口ＨＣ濃度（１点鎖線で示した）に対し、入口の触媒温度Ｔｃが４５０℃の場合の出口ＨＣ濃度は低レベルに抑えられており、この点は図８（ｂ）のＨＣ浄化効率線図より妥当と見做される。
【００５２】
ここで、入口の触媒温度Ｔｃが３００℃の場合、ＮＯｘ、ＨＣ、ＣＯは非パージ運転時（Ｌｔ＝３０ｓｅｃ）にはいずれも低レベルに保持され、パージ運転時（Ｔｒ＝２ｓｅｃ）にのみＮＯｘ、ＣＯの濃度が上がるが、これらは三元触媒Ｃ２で還元及び酸化処理されることとなり、全運転域で確実に、排気ガス浄化を高レベルで実行できる。しかも、触媒温度Ｔｃが３００℃を下回る場合には、パージ用燃料噴射ＰＰをしないので、活性化前のＮＯｘ触媒にＨＣ添加を行なって、返って排気ガス中のＨＣ濃度を上げることを排除でき、無駄な燃料消費を防止でき、結果的に触媒温度の変化にかかわらずＮＯｘ浄化効率及びＨＣ浄化効率を向上させることができる。
【００５３】
ステップＳ７でパルス噴射が実行された後、ステップｓ８で、パージ中フラグＰＦＬＧを「１」とし、この回の制御を終了させ、メインルーチンにリーンさせる。再度、ステップｓ１でＰＦＬＧ＝１を判断し、ステップｓ６に達すると、リッチパージ時間Ｔｒ経過前はステップｓ６、７、８を繰り返してリッチパルス噴射ＰＨ及びパージ用燃料噴射ＰＰを連続実施し、ＮＯｘ触媒Ｃ１からのＮＯｘ放出及び還元処理を継続する。
【００５４】
リッチパージ時間Ｔｒ経過後はステップｓ９で、ＰＦＬＧを「０」に戻し、ステップｓ１０でリッチパルス噴射ＰＨ及びパージ用燃料噴射ＰＰを解除し、メインルーチンに戻り、これにより、ＥＣＵ３６のメインルーチン内での燃料噴射制御手段Ａ０のみでの燃料噴射を実行することとなる。
このように、排気ガス浄化装置Ｍは、リッチ空燃比であっても、還元剤の追加供給を触媒温度Ｔｃに応じて停止することが可能となり、ＮＯｘ触媒Ｃ１が活性化される前の排気ガス中のＨＣ濃度の増加を防止できる。
【００５５】
ここでは、パージ制御手段Ａ３が還元剤の追加供給を膨張行程でリッチパルス噴射ＰＨとして行なうので、より的確に還元剤の追加供給を行なえ、ＮＯｘ触媒Ｃ１中のＮＯｘ放出、放出ＮＯｘの還元を促進できる。
図１の排気ガス浄化装置Ｍはリッチ空燃比において、還元剤の追加供給（パージ用燃料噴射ＰＰ）を触媒温度Ｔｃが閾値であるＴｃ１＝３００未満で停止していた。しかし、これに代えて、図７に示すように、触媒温度が２５０〜４００範囲で温度増加に応じて還元剤添加量（供給量）を順次増加させるように設定しても良い。
【００５６】
このように、パージ用燃料噴射ＰＰの供給量を触媒温度Ｔｃの増加に応じて増加させると、低温時に無駄な還元剤供給を抑えて排気ガス中のＨＣ濃度の増加を防止でき、高温に移動するに従い還元剤の追加供給量を増加させるので、パージパルス燃料噴射の切換えがスムーズになされ、しかも、低温側で比較的早く追加燃料供給でき、早期の活性化にも寄与できる。
【００５７】
図１の排気浄化触媒装置２６に代えて、図３に示すように排気浄化触媒装置２６ａを用いてもよい。この排気浄化触媒装置２６ａはケーシング２９ａ内に単一の担体３２ａを配備する。担体３２ａはその排気路上流側領域ａと排気路下流側領域ｂの担持層３４の成分と、そこに担持される貴金属の触媒成分３５を調整することで、図１のＮＯｘ触媒部Ｃ１、三元触媒部Ｃ２と同様のＮＯｘ触媒部Ｃ１’、三元触媒部Ｃ２’として形成される。このような図３に示す排気浄化触媒装置２６ａは図１の排気浄化触媒装置２６に代えて同様に使用でき、特に、単一の担体３２ａを用いるので、排気圧損失増大によるエンジン出力の低下を招くことがない。
【００５８】
更に、図１の排気浄化触媒装置２６ではＮＯｘ触媒Ｃ１の排気路下流側に三元触媒Ｃ２を縦配列したので、三元触媒がＮＯｘ触媒Ｃ１側で処理されていないＮＯｘ、ＣＯ、ＨＣを処理して、排気浄化触媒装置全体の浄化効率を向上させることができる。なお、図３の排気浄化触媒装置２６ａも図１の排気浄化触媒装置２６と同様の作用効果が得られる。
【００５９】
上述の実施形態では、排気ガス浄化装置Ｍを適用する内燃機関として、燃焼室内に燃料を直接噴射するようにした火花点火式の機関を例に挙げて説明したが、ディーゼルエンジンや、吸気管に燃料を噴射し混合気を燃焼室に導入する火花点火式のリーンバーンエンジンに適用することも可能であり、同様の作用効果を得ることができる。なお、本発明は上述した実施形態に限定されるものではなく、本発明の主旨を離脱しない範囲内で種々変更して実施しても構わない。例えば、温度検出手段は他の検出されたパラメータから温度を推定する温度推定手段でもよい。また、三元触媒がＢａを担持しない通常の三元触媒でも良いことは言うまでもない。
【００６０】
【発明の効果】
以上のように、本発明は、ＮＯｘ放出手段により排気空燃比をリッチ空燃比とした際に、パージ制御手段が還元剤の追加供給を触媒温度に応じて切換え制御することで、リッチ空燃比であっても、還元剤の追加供給を触媒温度に応じて停止することが可能となり、ＮＯｘ触媒装置が活性化される前の排気ガス中のＨＣ濃度の増加を防止できる。
【００６１】
請求項２の発明は、ＮＯｘ放出手段により排気空燃比をリッチ空燃比とした際に、触媒温度が設定温度未満では還元剤の追加供給を停止し、ＮＯｘ触媒装置が活性化される前の排気ガス中のＨＣ濃度の増加を防止し、設定温度以上で還元剤の追加供給を行って、ＮＯｘ触媒装置から放出されるＮＯｘの還元浄化を的確に行なうことができる。
【００６２】
請求項３の発明は、ＮＯｘ放出手段により排気空燃比をリッチ空燃比とした際に、還元剤の追加供給量を触媒温度の増加に応じて増加させるので、低温時に無駄な還元剤供給を抑えて排気ガス中のＨＣ濃度の増加を防止でき、高温に移動するに従い還元剤の追加供給量を増加させるので、燃料噴射の切換えがスムーズになされ、しかも、低温側で比較的早く追加燃料供給でき、早期の活性化にも寄与できる。
【００６３】
請求項４の発明は、ＮＯｘ触媒装置の排気路下流側の三元触媒はＮＯｘ触媒装置側で処理されていないＮＯｘ、ＣＯ、ＨＣを処理して、排気浄化触媒装置全体の浄化効率を向上させることができる。
【図面の簡単な説明】
【図１】本発明の一実施形態としての排気ガス浄化装置を装備したエンジンの概略構成図である。
【図２】図１の排気浄化触媒装置で用いる担体の部分拡大切欠断面図である。
【図３】図１の排気ガス浄化装置で用いる排気浄化触媒装置の変形例を示す。
【図４】図１の排気ガス浄化装置の駆動時のＮＯｘ濃度、空燃比、パルス噴射のモード説明図である。
【図５】図１の排気ガス浄化装置の駆動時のＮＯｘ濃度、ＣＯ濃度、ＴＨＣ（トータルＨＣ）濃度の経時特性線図である。
【図６】図１の排気ガス浄化装置が行なうパージ噴射処理ルーチンのフローチャートである。
【図７】本発明の排気ガス浄化装置の変形例で用いる触媒温度−ＨＣ添加量特性線図である。
【図８】従来の排気浄化触媒装置の概略図である。
【図９】図１の排気ガス浄化装置の特性値設定において参考として利用した排気浄化触媒装置の特性線図で、（ａ）はＮＯｘ浄化効率、（ｂ）はＨＣ浄化効率の特性線図である。
【符号の説明】
１エンジン
２６、２６ａ排気浄化触媒装置
２８温度センサ（温度検出手段）
３５貴金属の触媒成分
３６ＥＣＵ
Ａ０燃料供給制御手段
Ａ１ＮＯｘ放出手段
Ａ２還元剤供給手段
Ａ３パージ制御手段
Ｃ１ＮＯｘ触媒部
Ｃ２三元触媒部
Ｍ排気ガス浄化装置
Ｒｅ排気路
Ｔｃ触媒温度[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification device, and more particularly to an exhaust gas purification device that can efficiently purify NOx and HC.
[0002]
[Prior art]
In order to protect the environment, exhaust gas purification regulations for vehicles have been strengthened, and accordingly, there has been a demand for higher catalyst purification performance regardless of changes in the operating range of the internal combustion engine.
[0003]
By the way, in order to improve the fuel efficiency, a lean combustion internal combustion engine capable of combustion in an oxygen-excess atmosphere (lean air-fuel ratio) has been put into practical use. This lean burn internal combustion engine (lean burn engine) employs a NOx catalyst that stores NOx in exhaust gas and purifies NOx in exhaust gas during lean combustion. This NOx catalyst purifies NOx in exhaust gas by occluding NOx on the catalyst in an oxygen-excess atmosphere (lean air-fuel ratio), and adheres to the NOx when the oxygen concentration decreases (theoretical air-fuel ratio or rich air-fuel ratio). It has a function to release. That is, the NOx catalyst generates nitrate from the NOx in the exhaust in an oxygen-excess atmosphere, and thereby stores NOx, while the NOx catalyst stores the nitrate stored in the NOx catalyst and the CO in the exhaust in an atmosphere with a reduced oxygen concentration. It reacts to produce carbonate, which releases NOx.
[0004]
The NOx catalyst stores NOx on the catalyst in an oxygen-excess atmosphere during lean operation. However, when the NOx storage amount reaches the saturation amount by continuously performing lean operation, most of the NOx in the exhaust gas is exhausted. Will be discharged into the atmosphere. Therefore, before the NOx storage amount reaches the saturation amount, the air-fuel ratio is switched to the stoichiometric air-fuel ratio or rich air-fuel ratio, the exhaust is made into an atmosphere with a reduced oxygen concentration, NOx is released and reduced, and the NOx storage capacity of the NOx catalyst is increased. It tries to recover.
[0005]
In this operating range, the engine air-fuel ratio is switched to the stoichiometric air-fuel ratio or rich air-fuel ratio in order to recover the NOx storage capacity (CO is generated and exhausted, that is, supplied to the NOx catalyst) to release and reduce NOx. In this case, a part of the supplied CO is consumed to release the stored NOx, and the remaining CO is consumed to reduce the released NOx.
[0006]
However, the NOx release rate increases as the air-fuel ratio of the engine shifts to the rich side (as the CO amount increases), and in particular, released from the catalyst as the CO amount increases from near the stoichiometric air-fuel ratio. The amount of NOx released increases. For this reason, when there are few reducing agents (remaining CO, HC, etc. that did not contribute to NOx release) that reduce NOx released with respect to the increased NOx release amount, the released NOx remaining in the exhaust gas is reduced. It will be released into the atmosphere as it is.
[0007]
As described above, in the control in which the air-fuel ratio shifts from the lean side to the rich side and the theoretical air-fuel ratio or the shift to the rich side continues only slightly, the NOx release amount increases rapidly, and the reduced NOx <released. The relationship of NOx is not changed, and the NOx catalyst is not reduced and discharged downstream.
[0008]
As a solution to this, a purge operation in which expansion injection is added for a short time in addition to normal intake stroke injection is performed in order to enrich the exhaust air-fuel ratio in the catalyst at an early stage before the NOx storage amount of the NOx catalyst reaches the saturation amount. It is known. By performing this purge operation, it is possible to control to supply an increasing amount of reducing agent into the exhaust gas, and it is possible to release and reduce NOx from the NOx catalyst and improve NOx purification performance. -2114 (Patent Document 1).
[0009]
For example, in a direct injection lean burn engine, it is necessary to maintain high NOx trap performance during lean operation and NOx, HC, and CO purification performance during operation other than lean operation. Therefore, as shown in FIG. 8, an exhaust gas purification device 120 in which a NOx catalyst 100 and a three-way catalyst 110 are arranged in a vertical arrangement along the exhaust path is used. Note that the exhaust gas purification device disclosed in Patent Document 1 discloses a configuration in which a front three-way catalyst, a NOx catalyst, and a rear three-way catalyst are sequentially arranged in this order along an exhaust path.
[0010]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-2114
[0011]
[Problems to be solved by the invention]
In the exhaust gas purification device disclosed in FIG. 8 and Patent Document 1, the NOx purification performance can be improved. However, in this case, it is necessary that at least the NOx catalyst is activated, and when a reducing agent for HC is supplied from a reducing agent supply means (not shown) for the NOx purge process in a low temperature operation region that is not activated, This HC hardly contributes to the reduction, and the NOx emission amount cannot be reduced accurately. Moreover, the HC discharged as it is increases the HC emission concentration, leading to unnecessary fuel consumption.
[0012]
By the way, the inventors of the present application conducted an experiment considering that the NOx catalyst needs to explore the change characteristics of the NOx purification efficiency and the HC purification efficiency according to the change of the ambient temperature. Here, the tandem exhaust gas purification device of FIG. 8 is used, and the fuel injection drive by the fuel injection valve of the engine (not shown) is switched and adjusted to the lean injection pattern, the rich injection pattern, and the purge injection pattern, As a result, the air-fuel ratio was switched to lean, rich, and NOx purge.
[0013]
Here, the catalyst temperature was varied, and the NOx purification efficiency and the HC purification efficiency at each temperature were measured, and the characteristics shown in FIGS. 9A and 9B became clear.
As shown in FIG. 9 (a), the NOx purification efficiency of the exhaust gas purification apparatus is such that the catalyst temperature is 350 ° C. regardless of whether or not the NOx purge process is performed by adding a reducing agent (reference a is purge and reference b is not purged). Until then, the activation proceeds with almost the same purification efficiency due to the increase in temperature, and when it exceeds 350 ° C., the NOx trap performance is degraded due to the characteristics of the catalyst itself. In this case, it became clear that the NOx purifying efficiency is improved in the operating region exceeding 350 ° C. as compared with the case where the NOx purge process is not performed.
[0014]
As shown in FIG. 9 (b), the HC purification efficiency of the exhaust gas purification apparatus is such that the catalyst temperature increases from the low temperature side regardless of whether or not the NOx purge process is performed (reference a is purge and reference b is not purged). Has risen accordingly. In particular, the HC purification efficiency in a low temperature range exceeding 330 ° C. is lowered, and when a reducing agent (HC) is added here, the HC purification efficiency is further deteriorated. It has been clarified that when the temperature exceeds 330 ° C., the reducing agent (HC) is sufficiently consumed and the HC emission amount is suppressed.
[0015]
As described above, even if a reducing agent is added to increase the NOx purification efficiency in the exhaust gas purification device, the reduction function does not work accurately up to about 350 ° C., and further, the reducing agent does not operate until the HC purification efficiency exceeds 330 ° C. Since the HC purification efficiency is further deteriorated by adding (HC), it is preferable to suppress the addition of the reducing agent (HC) in the low temperature side region. On the other hand, the NOx purification efficiency is increased by adding a reducing agent at 350 ° C. or higher. When the temperature exceeds 330 ° C., the reducing agent (HC) is sufficiently consumed, and the HC purification efficiency is higher when the reducing agent (HC) is added. From the improvement, it was found that addition of a reducing agent (HC) is preferable in the high temperature side region.
[0016]
The present invention has been made on the basis of the above-described circumstances. By adjusting the reducing agent addition in the exhaust gas purifying apparatus according to the catalyst temperature, the NOx purification efficiency and the HC purification efficiency are affected by changes in the catalyst temperature. It is an object of the present invention to provide an exhaust gas purification device that can be improved without delay.
[0017]
[Means for Solving the Problems]
The invention of claim 1 is provided in the exhaust passage of the internal combustion engine and absorbs NOx in the exhaust when the exhaust air-fuel ratio is a lean air-fuel ratio, and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or the rich air-fuel ratio, NOx catalyst device for releasing and reducing the absorbed NOx, NOx releasing means for releasing NOx in the NOx absorption catalyst by setting the exhaust air-fuel ratio to a rich air-fuel ratio, and the exhaust air-fuel ratio is the stoichiometric air-fuel ratio Or a reducing agent supply means for additionally supplying a reducing agent for reducing the released NOx, a temperature detecting means for outputting the catalyst temperature of the NOx catalyst device, and a reducing agent supply means when the air-fuel ratio is rich. Purge control means for switching and controlling the additional supply of the reducing agent according to the catalyst temperature.
In this way, when the exhaust air-fuel ratio is made rich by the NOx releasing means, the purge control means switches and controls the additional supply of the reducing agent according to the catalyst temperature, so that even if the rich air-fuel ratio is the reduction The additional supply of the agent can be stopped according to the catalyst temperature, and an increase in the HC concentration in the exhaust gas before the NOx catalyst device is activated can be prevented.
Preferably, the purge control means performs the additional supply of the reducing agent during the expansion stroke. In this case, additional supply of the reducing agent can be performed more accurately, and NOx release and NOx reduction in the NOx absorption catalyst can be promoted.
[0018]
According to a second aspect of the present invention, in the exhaust gas purifying apparatus according to the first aspect, the purge control means stops the additional supply of the reducing agent by the reducing agent supply means when the catalyst temperature is lower than a preset temperature, An additional supply of the reducing agent is performed.
As described above, when the exhaust air-fuel ratio is made rich by the NOx releasing means, if the catalyst temperature is lower than the set temperature, the additional supply of the reducing agent is stopped and the exhaust gas before the activation of the NOx catalyst device is stopped. An increase in the HC concentration can be prevented, and the reducing agent can be additionally supplied at a temperature equal to or higher than the set temperature, so that the reduction and purification of NOx released from the NOx catalyst device can be performed accurately.
[0019]
According to a third aspect of the present invention, in the exhaust gas purifying apparatus according to the first or second aspect, the purge control means increases the additional supply amount of the reducing agent as the catalyst temperature increases.
As described above, when the exhaust air-fuel ratio is made rich by the NOx releasing means, the additional supply amount of the reducing agent is increased in accordance with the increase in the catalyst temperature. The increase in the concentration of HC in the fuel can be prevented, and the additional supply amount of the reducing agent is increased as it moves to a higher temperature. Therefore, the fuel injection can be smoothly switched, and the additional fuel can be supplied relatively quickly on the low temperature side. It can also contribute to activation.
[0020]
According to a fourth aspect of the present invention, in the exhaust gas purifying apparatus according to the first, second, or third aspect, a three-way catalyst is disposed on the downstream side of the exhaust passage of the NOx catalyst device.
Thus, the three-way catalyst on the downstream side of the exhaust passage of the NOx catalyst device can process NOx, CO, and HC that are not treated on the NOx catalyst device side, thereby improving the purification efficiency of the entire exhaust purification catalyst device. .
[0021]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an exhaust gas purifying apparatus M as an embodiment of the present invention and an internal combustion engine equipped with the apparatus. The internal combustion engine is an in-cylinder injection type 4-cycle multi-cylinder gasoline engine (hereinafter simply referred to as engine 1). The engine 1 has a combustion chamber 6 formed so as to be surrounded by a cylinder 3 in the cylinder block 2 and a piston 4 that slides up and down within the cylinder block 5 and a cylinder head 5 that overlaps and is integrally coupled to the cylinder block 2. The combustion chambers 6 are provided by the number of cylinders (only one is shown in FIG. 1). An electromagnetic fuel injection valve 7 is attached to the combustion chamber 6. The fuel injection valve 7 can perform each fuel injection in an intake stroke injection mode in which fuel is injected into the combustion chamber 6 in an intake stroke, a compression stroke injection mode in which fuel is injected in a compression stroke, and an expansion stroke injection mode in which fuel is injected in an expansion stroke. Is formed.
[0022]
The engine 1 can be operated at a stoichiometric air-fuel ratio (stoichiometric), an operation at a rich air-fuel ratio (rich air-fuel ratio operation), and an operation at a lean air-fuel ratio (lean air-fuel ratio operation). In the compression stroke injection mode, it is possible to operate at a super lean air / fuel ratio that is larger than the lean air / fuel ratio operation in the intake stroke.
A fuel injection valve 7, an intake valve 8, an exhaust valve 9, and a spark plug 11 are attached to the combustion chambers 6 at positions that do not interfere with each other.
[0023]
The piston 7 in the cylinder 3 is formed with a cavity 12 recessed in a hemispherical shape on the top surface of the piston. The cavity 12 cooperates with the suction port 13 so as to generate a clockwise reverse tumble flow S in FIG. An intake port is formed in the cylinder head 5 in a substantially upright direction for each cylinder. Each intake port (only one is shown in FIG. 1) 13 has an intake manifold 14 that forms an intake passage Ri, a surge tank 15, An intake pipe 17 having a throttle valve 16, an air cleaner 18, and an atmosphere side opening 19 are connected in this order, so that the atmosphere can be sucked along the intake path Ri.
[0024]
The throttle valve 16 is a drive-by-wire (DBW) type motor operated valve, and is provided with a throttle position sensor 21 for detecting the throttle opening θth. The engine 1 is provided with a crank angle sensor 22 that detects a crank angle, and the crank angle sensor 22 can detect an engine rotation speed Ne.
An exhaust port 23 is formed in the cylinder head 5 in a substantially horizontal direction for each cylinder. Each exhaust port (only one is shown in FIG. 1) 23 has an exhaust manifold 24 that forms an exhaust passage Re, an exhaust pipe ( (Exhaust passage) 25, an exhaust purification catalyst device 26 forming a main part of the exhaust gas purification device M, a downstream exhaust pipe 27, and a muffler (not shown) are connected in this order so that the exhaust can be discharged to the outside along the exhaust path Re. Is done.
[0025]
The exhaust pipe 25 immediately upstream of the exhaust purification catalyst device 26 is provided with a temperature sensor 28 as temperature detection means for detecting the exhaust temperature in order to estimate the catalyst temperature of the NOx catalyst device, and further on the upstream side, the air-fuel ratio A An air-fuel ratio sensor 30 for detecting / F is provided.
As shown in FIG. 1, the exhaust purification catalyst device 26 includes a cylindrical casing 29 having an enlarged inner diameter so as to be continuous with the exhaust pipe 25 and the downstream exhaust pipe 27, and a vertical orientation without deviation in the casing 29. It is formed by a NOx catalyst C1 and a three-way catalyst C2 that are arranged before and after.
[0026]
As shown in FIGS. 1 and 2, front and rear carriers 32 and 33 having a large number of through-holes 31 penetrating in the axial direction, and a refractory inorganic oxide formed on the inner surface defining each through-hole 31. And a noble metal catalyst component 35 held by the support layer 33.
Each of the carriers 32 and 33 is mainly composed of alumina, whereby the entire honeycomb structure including the support layer 34 in the vicinity of the inner surface of each through hole 31 is formed. A NOx trapping agent is mixed in the carrier layer 34 of the previous carrier 32, and a noble metal catalyst component 35 for promoting reduction is carried on the inner surface. An oxidation accelerator is mixed in the support layer 34 of the subsequent carrier 33, and a noble metal catalyst component 35 is supported on the inner surface.
[0027]
The carrier 32 of FIGS. 1 and 2 is formed by adopting an alumina mainly composed of K (potassium) having high heat resistance as a NOx trapping agent and having high NOx trapping performance. 34 is formed by adopting alumina mainly composed of Ba (barium) having high HC and CO oxidation performance.
Further, Pt (platinum) is supported on the inner wall surfaces of the support layers 34 of the carriers 32 and 33 as a noble metal catalyst component 35 having high NOx reduction treatment and HC oxidation treatment performance. Here, the precious metal catalyst component 35 is preferably larger in order to enhance the performance of NOx reduction treatment and HC oxidation treatment, but the NOx trapping agent (potassium) NOx adsorption, release, and reduction functions on the NOx catalyst C1 side. An appropriate amount is supported within a range in which the above is not suppressed.
[0028]
Such an upstream NOx catalyst C1 of the exhaust purification catalyst device 26 temporarily stores NOx when the exhaust air-fuel ratio that is an oxygen-excess atmosphere is a lean air-fuel ratio, the oxygen concentration decreases, and the exhaust air-fuel ratio becomes stoichiometric. The fuel cell has a NOx release / reduction function that releases NOx and reduces it to N2 (nitrogen) or the like at the fuel ratio or rich air-fuel ratio, that is, in a reducing atmosphere in which CO is present.
The three-way catalyst C2 on the downstream side has a three-way function of oxidizing CO and HC in the exhaust gas and reducing and purifying NOx in a stoichiometric air-fuel ratio and rich atmosphere. The three-way catalyst C2 is released from the NOx catalyst C1, and also reduces NOx that could not be reduced in the same part.
[0029]
The vehicle is provided with a controller (hereinafter simply referred to as an ECU 36) that is an engine control means. The ECU 36 includes an input / output device, a storage device that stores a control program, a control map, and the like, a central processing unit, timers, and counters. Is provided. The ECU 36 controls the exhaust purification catalyst device 26 including the engine 1. The detection information of the various sensors is input to the ECU 36, and the ECU 36 performs fuel supply control and ignition control based on the detection information of the various sensors, that is, the ignition timing and the like including the fuel injection mode and the fuel injection amount. The fuel injection valve 7 and the spark plug 11 are driven and controlled.
[0030]
The fuel supply control means A0 of the ECU 36 injects fuel after the middle of the compression stroke when in the steady operation range, collects a small amount of fuel only in the vicinity of the spark plug 11, and sets only the vicinity of the spark plug 11 to the theoretical air-fuel ratio or rich. By setting the air-fuel ratio A / F to be low, fuel consumption is suppressed while realizing stable stratified combustion (stratified super lean combustion). In order to obtain a high output, the fuel from the fuel injection valve 7 is injected in the intake stroke so as to be uniformly dispersed throughout the combustion chamber 6, and the inside of the combustion chamber 6 has a theoretical air fuel ratio (approximately A / F: 15). And lean air-fuel is made into this air-fuel mixture state, premixed combustion is performed, and high output is obtained.
[0031]
In this fuel supply control means A0, the target in-cylinder pressure corresponding to the engine load, that is, the target average effective pressure Pe, is determined based on the throttle opening θth from the throttle position sensor 21 and the engine rotational speed Ne from the crank angle sensor 22. Further, the fuel injection mode is set from a map (not shown) according to the target average effective pressure Pe and the engine rotational speed Ne.
[0032]
For example, when both the target average effective pressure Pe and the engine speed Ne are small, the fuel injection mode is set to the compression stroke injection mode, and fuel is injected in the compression stroke. On the other hand, when the target average effective pressure Pe increases or the engine speed Ne increases, the fuel injection mode is set to the intake stroke injection mode. Then, the target air-fuel ratio O which becomes a control target in each fuel injection mode from the target average effective pressure Pe and the engine rotational speed Ne. _{A / F} For example, the lean air-fuel ratio se1 is operated by the pulse injection PL in FIG. 4, and the rich air-fuel ratio se2 is operated by the pulse injection PH indicated by the broken line in FIG. The amount is the target air-fuel ratio O _{A / F} To be determined.
[0033]
By the way, in the NOx catalyst C1 of the exhaust purification catalyst device 26, when the air-fuel ratio in the exhaust gas is a lean air-fuel ratio, NOx in the exhaust is occluded as nitrate and the exhaust gas is purified. On the other hand, when the air-fuel ratio in the exhaust gas is the stoichiometric air-fuel ratio or the rich air-fuel ratio, the nitrate stored in the NOx catalyst C1 reacts with the CO in the exhaust gas to generate carbonate and to release NOx. Accordingly, when NOx occlusion in the NOx catalyst C1 proceeds, a rich purge that enriches the air-fuel ratio is performed to reduce the oxygen concentration, supply CO to the NOx catalyst C1, and release and reduce NOx from the NOx catalyst C1. Thus, the NOx occlusion function can be maintained.
[0034]
Therefore, the ECU 36 is provided with functions of NOx releasing means A1, reducing agent supply means A2, and purge control means A3 in addition to the fuel supply control means A0. The NOx releasing means A1 lowers the oxygen concentration in the exhaust (rich air / fuel ratio) and releases NOx from the NOx catalyst C1. The reducing agent supply means A2 has a target air-fuel ratio O _{A / F} When P is a stoichiometric air-fuel ratio or rich, a purge fuel injection PP for supplying additional reducing agent (HC) for reducing released NOx is designated. Therefore, at the time of the purge command, the ECU 36 designates the rich air-fuel ratio se2 by the NOx releasing means A1, and designates the fuel injection PP for purge that is the additional fuel by the reducing agent supply means A2, so that the super-rich air-fuel ratio se3 is designated. Thus, the operation with the pulse injection PH + PP in FIG. 4 is performed. The purge control means A3 switches and controls the execution of the purge fuel injection PP for additionally supplying the reducing agent (HC) by the reducing agent supply means A2 in accordance with the catalyst temperature Tc obtained from the high temperature sensor 28.
[0035]
Here, the NOx releasing means A1 outputs the target air-fuel ratio as the rich air-fuel ratio during the rich purge time Tr (see, for example, 2 sec) to release NOx from the NOx catalyst C1, and then the lean continuation time Lt (see FIG. 5 (for example, 30 sec), a rich purge function of returning the exhaust air-fuel ratio to a lean air-fuel ratio is provided.
The purge control means A3 switches and controls the additional supply of the reducing agent (purge fuel injection PP) according to the catalyst temperature Tc. That is, the purge control unit A3 stops the additional supply of the reducing agent by the reducing agent supply unit A2 when the catalyst temperature Tc is lower than the set temperature Tc1, and performs the additional supply of the reducing agent above the set temperature Tc1.
[0036]
Here, the set temperature Tc1 was set to 300 ° C. The reason for this is that, as described in FIGS. 9A and 9B, the NOx catalyst C1 of the exhaust purification catalyst device 26 does not function properly until the ambient temperature reaches 350 ° C., and the reducing agent ( The addition of HC) deteriorates the HC purification efficiency until it exceeds 330 ° C, and further, the NOx purification efficiency is increased by adding a reducing agent (HC) at 350 ° C or higher. However, it is considered that each point is improved when a reducing agent (HC) is added, and that early activation is achieved. The set temperature Tc1 = 300 ° C. varies depending on the capacity of the NOx catalyst C1, the carrier 32 constituting the NOx catalyst C1, the trapping agent of the support layer 34, and the catalyst component 35 of the noble metal, and is 300 to 350 ° C. It will be appropriately modified within the range.
Note that the three-way catalyst C2 downstream of the NOx catalyst C1 has its catalyst temperature substantially linked to the NOx catalyst C1, and is activated almost simultaneously.
[0037]
The operation of the above-described exhaust purification catalyst device 26 will be described.
Here, when the lean pulse injection PL is executed, the NOx catalyst C1 absorbs NOx in the carrier layer 34 and performs NOx purification. Further, when the rich pulse injection PH is executed, the NOx catalyst C1 on the front side of the three-way catalyst C2 starts releasing CO when the exhaust gas reaches near the stoichiometric air-fuel ratio, but only reduces the released NOx. Since there are few reducing agents (remaining CO and HC, etc.), the released NOx> NOx to be reduced, and NOx flows to the three-way catalyst C2, where it is purified together with CO and HC.
[0038]
On the other hand, when the accumulated operation time of the lean operation exceeds the lean continuation time Lt, the purge control means A3 of the ECU 36 determines that the NOx catalyst is activated only when the catalyst temperature Tc exceeds the set temperature Tc1 = 300 ° C. The driving of the NOx releasing means A1 and the reducing agent supply means A2 is permitted. Here, the NOx releasing means A1 sets the target air-fuel ratio to a rich air-fuel ratio and gives a rich pulse injection PH instruction. Moreover, the reducing agent supply means A2 that has received the rich purge signal instructs the purge fuel injection PP so that the reducing agent supply continues until the expansion stroke. As a result, the fuel injection drive circuit 37 adds the fuel injection valve 7 to the rich pulse injection PH and performs the fuel injection drive in a pattern in which the drive of the purge fuel injection PP is continued (see pulse injection in FIG. 4). Then, the counter is set and driven to perform the fuel injection driving.
[0039]
As a result, unburned HC (reducing agent) that is injected in the expansion stroke and does not contribute to combustion is discharged from the combustion chamber to the exhaust passage Re and becomes a reducing agent for HC and CO, thereby NOx released from the NOx catalyst C1 is reduced. This is rapidly increased (see NOx concentration in FIG. 5), and this is sufficiently reduced by the noble metal catalyst component 35, and even the three-way catalyst C2 purifies NOx, CO, and HC with a relatively large amount of the noble metal catalyst component 35. Therefore, NOx, CO, and HC can be prevented from being released into the atmosphere, and the problem that NOx released from the NOx catalyst C1 is directly released into the atmosphere is eliminated.
[0040]
The NOx purge control that is the NOx release reduction control described above will be described with reference to the flowchart of FIG.
It is assumed that the NOx purge control is reached during the fuel supply control in the main routine (not shown) of the ECU 36.
[0041]
In the fuel supply control of the main routine (not shown), the fuel injection drive circuit 37 counts the injection timing and the injection drive time with a counter (not shown) along the injection pattern instructed by the ECU 36, and the fuel injection valve 7 is injected with fuel. Operate. For example, the target air-fuel ratio O _{A / F} 4 is a lean air-fuel ratio (symbol se1 shown in FIG. 4), a lean operation command is issued, and the fuel injection drive circuit 37 sets a counter (not shown) to perform lean pulse injection PL, and the lean pulse shown in FIG. Drive by injection PL. Furthermore, the target air-fuel ratio O _{A / F} When the lean air-fuel ratio is changed from the lean air-fuel ratio to the stoichiometric and rich air-fuel ratio, the counter is set to perform the stoichiometric and rich pulse injection PH, and the rich pulse injection PH shown in FIG. 4 is performed. At this time, the air-fuel ratio A / F near the position indicated by the broken line in FIG. 4 continues.
[0042]
If there is a purge injection control instruction in the middle of the fuel injection control means A0 in the main routine, the purge injection process of FIG. 6 is reached.
Here, if it is the first time of control, the purging flag PFLG (described later) is “0” in step s1, and the process proceeds to step S2. If PFLG = 1, the process proceeds to step s6. In step S2, it is determined whether or not the temperature T of the three-way catalyst C2 is equal to or higher than Ts (activation temperature) (estimated by the exhaust temperature value detected by the high temperature sensor 28), and the NOx catalyst C1 and the three-way catalyst C2 are detected. When it is determined that the temperature T is equal to or higher than Ts (that is, when the three-way catalyst C2 is active), it is determined in step S3 whether the lean mode duration t1 is equal to or longer than the lean duration Lt.
[0043]
The lean duration Lt is set so as to satisfy the following requirements (1) to (3).
(1) It is desirable that the NOx is not stored in the NOx catalyst C1 while the lean operation is continued, and is not longer than the time until it is released into the atmosphere.
(2) When the lean time continues for a long time (three-way), the catalyst temperature decreases. Therefore, the lean duration Lt is preferably equal to or less than a predetermined value.
(3) In order to prevent the NOx purge dead time tu (see FIG. 4) required to reach the stoichiometric state from the lean at the start-up of the NOx purge, the lean continuation time Lt should be a predetermined value or less in order to prevent the fuel consumption from deteriorating. Is preferred.
[0044]
Here, 30 seconds is set as the optimum lean duration Lt under the above three conditions.
When step S4 is reached, it is determined here whether or not the catalyst temperature Tc exceeds the set temperature Tc1 = 300 ° C., and only when it exceeds, that is, when the NOx catalyst is activated, the process proceeds to step s5.
[0045]
In step S5, assuming that the NOx catalyst is activated and the start condition of the control (NOx purge) for releasing and reducing NOx occluded in the NOx catalyst C1 is satisfied, the rich purge time Tr and purge for performing the fuel injection PP for purge here A pulse injection time Ti that is an injection time of the fuel injection PP is set.
[0046]
The rich purge time Tr is set based on the product of the map value of the exhaust flow rate, for example, the intake air amount Qa, and the degree of deterioration of the NOx catalyst C1, for example, the map value of the travel distance Km (from the integrated distance detector 36). . The rich purge time Tr is set to about 1 to 5 seconds, for example, and as the travel distance becomes longer, the region n where the NOx release speed is high in the NOx release characteristic diagram as shown in FIG. 4 becomes narrower. By shortening, the deterioration of fuel consumption and the release of unburned HC and CO are suppressed.
[0047]
The pulse injection time Ti is set to, for example, about 0.1 to 1 second, and is set to be as short as about 1/2 as the travel distance becomes longer, that is, as the deterioration degree becomes higher. The reason why the pulse injection time Ti is shortened is that when the degree of deterioration of the NOx catalyst 15 increases, the NOx release characteristics change, and the region n (see FIG. 4) where the NOx release rate is fast is narrowed. It suppresses the deterioration of fuel consumption and the release of unburned HC.
[0048]
In step S5, the rich purge time Tr and the pulse injection time Ti are set. After the count is started, it is determined whether the rich purge time Tr has been reached in step S6. Before reaching the rich purge time Tr, the pulse injection time Ti is determined in step S7. A purge fuel injection PP is commanded.
As a result, when the rich purge time Tr is reached, the fuel injection drive circuit 37 drives the purge fuel injection PP immediately after the rich pulse injection PH and continues the fuel injection in a pattern (see the pulse injection amount in FIG. 4). The fuel injection driving is performed by driving the valve 7 and continuing the pulse injection PH + PP.
[0049]
Thereby, a reducing agent is added to the combustion chamber 6. That is, the purge fuel injection PP after the rich pulse injection PH is preferably from the middle stage of the expansion stroke to the early stage of the exhaust stroke, particularly the latter stage of the expansion stroke. Fuel (reducing agent) is supplied to the exhaust passage, and this rapidly releases the NOx adsorbed on the NOx catalyst C1 (see the NOx concentration in FIGS. 4 and 5), and the released NOx is a precious metal catalyst component. It is reduced by the action of 35.
[0050]
In this case, as shown in the NOx concentration diagram of FIG. 5, the NOx concentration at the outlet (shown by a one-dot chain line) when the catalyst temperature Tc at the inlet is 300 ° C. does not show a relatively excessive concentration increase. It is clear that the purification efficiency is maintained at a high level. On the other hand, when the catalyst temperature Tc at the inlet is 450 ° C., the outlet NOx concentration (shown by the solid line) is higher than that at 300 ° C., and the NOx purification efficiency is lowered. As shown in the graph of CO concentration (%), the outlet CO concentration (%) when the catalyst temperature Tc at the inlet is 300 ° C. is extremely low, and when 450 ° C. (shown by the solid line), A predetermined concentration is shown.
[0051]
Further, as shown in the HC concentration (%) diagram of FIG. 5, the inlet catalyst temperature Tc is 450 ° C. with respect to the outlet HC concentration (indicated by a one-dot chain line) when the inlet catalyst temperature Tc is 300 ° C. In this case, the outlet HC concentration is suppressed to a low level, and this point is considered to be appropriate from the HC purification efficiency diagram of FIG.
[0052]
Here, when the catalyst temperature Tc at the inlet is 300 ° C., NOx, HC and CO are all kept at a low level during the non-purge operation (Lt = 30 sec), and NOx only during the purge operation (Tr = 2 sec). The concentration of CO increases, but these are reduced and oxidized by the three-way catalyst C2, and exhaust gas purification can be reliably performed at a high level in the entire operation region. Moreover, when the catalyst temperature Tc is lower than 300 ° C., the purge fuel injection PP is not performed, so that it is possible to eliminate the increase of the HC concentration in the exhaust gas by adding HC to the NOx catalyst before activation and returning it. As a result, wasteful fuel consumption can be prevented, and as a result, NOx purification efficiency and HC purification efficiency can be improved regardless of changes in the catalyst temperature.
[0053]
After the pulse injection is executed in step S7, the purge flag PFLG is set to “1” in step s8, the control of this time is ended, and the main routine is made lean. Again, PFLG = 1 is determined in step s1, and when step s6 is reached, steps s6, 7 and 8 are repeated before the rich purge time Tr elapses, and the rich pulse injection PH and the purge fuel injection PP are continuously performed. The NOx release from the catalyst C1 and the reduction process are continued.
[0054]
After the rich purge time Tr has elapsed, PFLG is returned to “0” in step s9, rich pulse injection PH and purge fuel injection PP are canceled in step s10, and the process returns to the main routine. The fuel injection is executed only by the fuel injection control means A0.
Thus, even if the exhaust gas purification device M has a rich air-fuel ratio, it becomes possible to stop the additional supply of the reducing agent according to the catalyst temperature Tc, and the exhaust gas before the NOx catalyst C1 is activated. An increase in the concentration of HC can be prevented.
[0055]
Here, since the purge control means A3 performs the additional supply of the reducing agent as the rich pulse injection PH in the expansion stroke, the additional supply of the reducing agent can be performed more accurately, and the NOx release in the NOx catalyst C1 and the reduction of the released NOx are promoted. it can.
In the rich air-fuel ratio, the exhaust gas purifying apparatus M in FIG. 1 stops the additional supply of the reducing agent (purge fuel injection PP) when the catalyst temperature Tc is less than Tc1 = 300, which is the threshold value. However, instead of this, as shown in FIG. 7, the reducing agent addition amount (supply amount) may be set to increase sequentially as the temperature increases in the range of the catalyst temperature of 250 to 400.
[0056]
As described above, when the supply amount of the purge fuel injection PP is increased in accordance with the increase in the catalyst temperature Tc, it is possible to suppress an unnecessary supply of reducing agent at a low temperature and prevent an increase in the HC concentration in the exhaust gas and move to a high temperature. Accordingly, the additional supply amount of the reducing agent is increased, so that the purge pulse fuel injection can be smoothly switched, and the additional fuel can be supplied relatively quickly on the low temperature side, thereby contributing to early activation.
[0057]
Instead of the exhaust purification catalyst device 26 of FIG. 1, an exhaust purification catalyst device 26a may be used as shown in FIG. This exhaust purification catalyst device 26a is provided with a single carrier 32a in a casing 29a. The carrier 32a adjusts the components of the support layer 34 in the exhaust passage upstream region a and exhaust passage downstream region b and the noble metal catalyst component 35 supported on the support layer 34, whereby the NOx catalyst part C1, three of FIG. The NOx catalyst part C1 ′ and the three-way catalyst part C2 ′ are the same as the original catalyst part C2. Such an exhaust purification catalyst device 26a shown in FIG. 3 can be used in the same manner instead of the exhaust purification catalyst device 26 of FIG. 1, and in particular, since a single carrier 32a is used, the engine output is reduced due to an increase in exhaust pressure loss. There is no invitation.
[0058]
Further, in the exhaust purification catalyst device 26 of FIG. 1, since the three-way catalyst C2 is vertically arranged on the downstream side of the exhaust passage of the NOx catalyst C1, the three-way catalyst treats NOx, CO, and HC that are not treated on the NOx catalyst C1 side. Thus, the purification efficiency of the entire exhaust purification catalyst device can be improved. The exhaust purification catalyst device 26a of FIG. 3 can also obtain the same effects as the exhaust purification catalyst device 26 of FIG.
[0059]
In the above-described embodiment, the spark ignition type engine in which the fuel is directly injected into the combustion chamber is described as an example of the internal combustion engine to which the exhaust gas purification device M is applied. The present invention can also be applied to a spark ignition type lean burn engine in which fuel is injected and an air-fuel mixture is introduced into a combustion chamber, and similar effects can be obtained. The present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the spirit of the present invention. For example, the temperature detection unit may be a temperature estimation unit that estimates the temperature from other detected parameters. Needless to say, the three-way catalyst may be an ordinary three-way catalyst not supporting Ba.
[0060]
【The invention's effect】
As described above, according to the present invention, when the exhaust air-fuel ratio is changed to the rich air-fuel ratio by the NOx releasing means, the purge control means switches and controls the additional supply of the reducing agent according to the catalyst temperature. Even if it exists, it becomes possible to stop additional supply of a reducing agent according to catalyst temperature, and can prevent the increase in HC concentration in exhaust gas before a NOx catalyst device is activated.
[0061]
According to the second aspect of the present invention, when the exhaust air-fuel ratio is made rich by the NOx releasing means, if the catalyst temperature is lower than the set temperature, the additional supply of the reducing agent is stopped and the exhaust before the NOx catalyst device is activated. It is possible to prevent an increase in the HC concentration in the gas and perform additional reduction and purification of NOx released from the NOx catalyst device by additionally supplying a reducing agent at a set temperature or higher.
[0062]
According to the third aspect of the present invention, when the exhaust air-fuel ratio is made rich by the NOx releasing means, the additional supply amount of the reducing agent is increased in accordance with the increase in the catalyst temperature. Therefore, the increase in the HC concentration in the exhaust gas can be prevented, and the additional supply amount of the reducing agent is increased as it moves to a higher temperature, so that the fuel injection can be switched smoothly and the additional fuel can be supplied relatively quickly on the low temperature side. It can also contribute to early activation.
[0063]
In the invention of claim 4, the three-way catalyst on the downstream side of the exhaust passage of the NOx catalyst device treats NOx, CO, and HC that are not treated on the NOx catalyst device side, thereby improving the purification efficiency of the entire exhaust purification catalyst device. be able to.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an engine equipped with an exhaust gas purification apparatus as one embodiment of the present invention.
FIG. 2 is a partially enlarged cutaway cross-sectional view of a carrier used in the exhaust purification catalyst device of FIG.
3 shows a modification of the exhaust purification catalyst device used in the exhaust gas purification device of FIG. 1. FIG.
4 is an explanatory diagram of NOx concentration, air-fuel ratio, and pulse injection mode when the exhaust gas purifying apparatus of FIG. 1 is driven. FIG.
5 is a time characteristic diagram of NOx concentration, CO concentration, and THC (total HC) concentration when the exhaust gas purifying apparatus of FIG. 1 is driven. FIG.
6 is a flowchart of a purge injection processing routine performed by the exhaust gas purification device of FIG.
FIG. 7 is a catalyst temperature-HC addition amount characteristic diagram used in a modification of the exhaust gas purifying apparatus of the present invention.
FIG. 8 is a schematic view of a conventional exhaust purification catalyst device.
9 is a characteristic diagram of an exhaust purification catalyst device used as a reference in setting the characteristic values of the exhaust gas purification device of FIG. 1, (a) is a NOx purification efficiency, and (b) is a characteristic diagram of HC purification efficiency. is there.
[Explanation of symbols]
1 engine
26, 26a Exhaust purification catalyst device
28 Temperature sensor (temperature detection means)
35 Precious metal catalyst components
36 ECU
A0 Fuel supply control means
A1 NOx release means
A2 Reducing agent supply means
A3 Purge control means
C1 NOx catalyst section
C2 Three-way catalyst part
M exhaust gas purification device
Re exhaust passage
Tc catalyst temperature

Claims

内燃機関の排気路に設けられ排気空燃比がリーン空燃比である時に排気中のＮＯｘを吸収し、排気空燃比が理論空燃比又はリッチ空燃比であるときに、上記吸収したＮＯｘを放出して還元処理するＮＯｘ触媒装置と、
上記排気空燃比をリッチ空燃比とすることで上記ＮＯｘ吸収触媒中のＮＯｘを放出させるＮＯｘ放出手段と、
上記排気空燃比が理論空燃比又はリッチ空燃比であるときに、上記放出されたＮＯｘを還元する還元剤を追加供給する還元剤供給手段と、
上記ＮＯｘ触媒装置の触媒温度を出力する温度検出手段と、
上記還元剤供給手段による還元剤の追加供給を上記触媒温度に応じて切換え制御するパージ制御手段と、を具備する排気ガス浄化装置。NOx in the exhaust gas is absorbed when the exhaust air-fuel ratio is a lean air-fuel ratio provided in the exhaust path of the internal combustion engine, and when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or the rich air-fuel ratio, the absorbed NOx is released. A NOx catalyst device for reduction treatment;
NOx releasing means for releasing NOx in the NOx absorption catalyst by setting the exhaust air / fuel ratio to a rich air / fuel ratio;
A reducing agent supply means for additionally supplying a reducing agent for reducing the released NOx when the exhaust air-fuel ratio is a stoichiometric air-fuel ratio or a rich air-fuel ratio;
Temperature detecting means for outputting the catalyst temperature of the NOx catalyst device;
An exhaust gas purification device comprising: purge control means for switching and controlling additional supply of the reducing agent by the reducing agent supply means according to the catalyst temperature.

請求項１記載の排気ガス浄化装置において、
上記パージ制御手段は上記触媒温度が設定温度未満では上記還元剤供給手段による還元剤の追加供給を停止し、設定温度以上で還元剤の追加供給を行なうことを特徴とする排気ガス浄化装置。The exhaust gas purification device according to claim 1,
The exhaust gas purifying apparatus according to claim 1, wherein the purge control unit stops the additional supply of the reducing agent by the reducing agent supply unit when the catalyst temperature is lower than the set temperature, and performs the additional supply of the reducing agent at a temperature equal to or higher than the set temperature.

請求項１又は２記載の排気ガス浄化装置において、
上記パージ制御手段は上記還元剤の追加供給量を上記触媒温度の増加に応じて増加させることを特徴とする排気ガス浄化装置。The exhaust gas purification device according to claim 1 or 2,
The exhaust gas purifier according to claim 1, wherein the purge control means increases the additional supply amount of the reducing agent in accordance with an increase in the catalyst temperature.

請求項１、２又は３に記載の排気ガス浄化装置において、
上記ＮＯｘ触媒装置の排気路下流側に三元触媒が配置されたことを特徴とする排気ガス浄化装置。The exhaust gas purification device according to claim 1, 2, or 3,
An exhaust gas purification device, wherein a three-way catalyst is disposed downstream of the exhaust passage of the NOx catalyst device.